Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and non-volatile/flash memory.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems.
Each cell in a non-volatile memory device can be erased and programmed. For example, memory cells can be programmed as a single bit per cell (e.g., as in the case with a single level cell—SLC) or multiple bits per cell (e.g., as in the case with a multilevel cell—MLC). Each cell's threshold voltage (Vth) determines the data that is stored in the cell. For example, in an SLC, a Vth of 0.5V might indicate a programmed cell while a Vth of −0.5V might indicate an erased cell. MLC devices can have multiple threshold voltages representing multiple programmed states.
Once a memory cell goes through an erase operation, it is erase verified. The purpose of the erase verify operation is to determine that the cell's Vth is below a maximum voltage level (e.g., −1V). The typical method for performing an erase verify is to apply an erase verify voltage (e.g., 0V) as Vw1 to all of the memory cell control gates of a series string of memory cells. This is typically accomplished by substantially simultaneously biasing all of the access lines (e.g., word lines) of a series string. A Vth of the series string is then measured and compared to a reference Vth.
The typical prior art erase verification provides an “average” threshold voltage of all of the memory cells in the series string of memory cells being measured. It is possible that one or more of the memory cells undergoing the erase verification has a Vth that is more positive than the reference threshold. Thus, the series string might pass the verification process and still have a memory cell that is not completely erased. For example, one or more memory cells could have a Vth of −1.3V instead of the target −2V that indicates erasure.
In order to accommodate the possible variations in erase verification, larger erase margins are used during verification. In other words, a range of voltages is used during verification instead of one voltage. Thus, if a memory device is erased to a voltage within a certain voltage range, it is considered erased. In an MLC device, this has the drawback of using larger portions of the limited voltage range that can be assigned to different states of the memory device.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an improved method for verifying an erased state of a memory cell.